Prior art panoramic dental X-ray machines are well known. Some provide a continuous image of the dental arch area and commonly employ an X-ray source and X-ray film both optically aligned with each other and supported on a rotatable carrying arm which orbits a patient situated in the path of the X-ray beams. The patient may remain stationary or be transported in a patient chair in accordance with various type drive mechanisms in order to simulate the generally elliptical shape of the human dental arch. The continuous image radiograph provides the dentist with a panoramic view of the teeth and associated structures and is therefore a useful diagnostic aid in many phases of dental practice.
Various other prior art apparatus provide a discontinuous, or split image panoramic radiograph which possesses certain advantages. Here, the dentist is presented with additional interpretive information since two distinctly different views of the incisors, or centrals area are provided. Additionally, overlying spinal shadows which would be cast over the central-bicuspid region are eliminated since X-rays are not generated when the spine is aligned with the X-ray source and film.
Regardless of the type radiographic image to be obtained, i.e., continuous or discontinuous, compensation is usually made for the fact that the curvature of the desired area of focus is generally not a true circle or ellipse. Thus, the rate of film travel must be varied in accordance with the rate of travel of the X-ray source about the patient's head in order that the radiological projections occupy a distance on the film commensurate with the linear distance of a curved structure being X-rayed, such as a typical dental arch.
In U.S. Pat. No. 2,798,958, apparatus is disclosed for varying the rate of film travel relative to the rate of travel of the X-ray source. The X-ray source and film carrier are both supported by a single member permitting both the X-ray source and film carrier to orbit the patient at an uniform rate of travel. Means are also disclosed for reorienting the patient after comletion of one-half of the excursion cycle in order to relocate the center of the axis of rotation with respect to the patient's head prior to X-raying the other one-half of the dental arch in order to provide the discontinuous, or split radiographic images.
In U.S. Pat. No. 3,045,118, apparatus is disclosed which automatically shifts the patient in order that the line of sight between the X-ray source and film bypasses the patient's spinal column and permits X-raying of the other half of the dental arch. Apparatus is also disclosed therein for continuously moving an X-ray source and extra-oral film holder about the patient.
In U.S. Pat. No. 3,636,349, structure is disclosed for revolving the X-ray source and film carrier about the head of a patient who remains fixed in position while the centerline of the orbit continuously moves through an arcuate path approximating the arch of the patient's teeth. The patent further discloses film carrier means which may be used advantageously in the practice of the present invention.
In U.S. Pat. No. 4,125,774, improved X-Y drive mechanism permits both continuous and discontinuous radiographic images of the dental arch area to be accurately portrayed.
Thus, the prior art discloses various types of structures, apparatus and mechanisms for orbiting the X-ray source-X-ray film (tubehead-camera) assemblies in circular or arcuate paths; for varying film-travel speed in accordance with tubehead-camera assembly movements; for shifting the patient in a chair; and for providing continuous or discontinuous type radiographic images.
In each of the aforedescribed prior art systems, the patient is subjected to radiation doses of sufficiently high intensify in order to project the desired images adequately onto the X-ray film, necessitating a supply of considerable power to the X-ray source or tubehead. The resultant radiographs lacked good static and dynamic resolution, providing approximately 5 line pairs/mm for static applications and only 3 to 4 line pairs/mm for dynamic applications. Our laboratory tests have confirmed that the prior art intensifying screens limit the total system resolution and hence, when the screens are eliminated as in the present invention, higher resolutions are obtained as the film has much higher resolution capabilities. It is noted that the unaided human eye is capable of distinguishing resolutions approaching about 7 line pairs/mm.
Further, in each of the prior art systems abovementioned, the X-ray film is required to be sandwiched between conventional intensifying screens which permit the film to obtain the image in a shorter time and with less X-ray exposure to the patient. The screens and film move as a unit past the camera slot. These intensifying screens are expensive and reusable but are easily damaged. For example, cracks, fissures, and embedded dirt in the screens provide false images, as do bent screens; electrostatic charges which have developed on the screen surfaces create lightning-like patterns on the radiograph, and the like. To the best knowledge of the inventor, all current panoramic dental X-ray machines employ intensifying screens with the X-ray film.
Attempts have recently been made to employ image intensification devices in conjunction with associated electronic peripheral components and equipment to substantially lessen the overall radiation dosages to which a patient is subjected without any concomitant sacrifice in contrast, resolution, or physical dimensions of the final radiograph. It is appreciated that radiographs of adequate physcial dimensions are considered necessary if meaningful information therefrom is to be consistently obtained by a dentist.
In accordance with the above, radiation exiting the patient may be directed into a system of components including a suitable image intensifying device in order to provide a real-time oscilloscope display for subsequent photographing thereof. The image intensifiers so employed, in the main, were very expensive, and sufficiently large in order to produce a proportionally large real-time display, the radiograph of which would require no electronic or optical "blow-up". It should be appreciated that blown-up radiographs amplify the already rather poorly resolved image as well as introduce geometric distortions thereunto, both of which could lead to a faulty diagnosis. Additionally, the use of such prior art image intensifying devices generally required the presence of cathode ray tubes, electronic amplifiers and sweeps, synchronous circuits, and the like, the totality of which still yielded undesirably low resolution radiographs.